The principal aim of this study is to determine the biochemical and pathological changes that occur in the lungs in the animals after continous and intermittent exposures to hyperoxia and hyperbaric oxygen. Particular emphasis will be placed on studies during recovery periods following continuous and intermittent exposures to oxygen concentrations ranging from 70% O2 at ambient pressure to 2.0 atmospheres absolute (ATA) of 100% oxygen. The biochemical assays to be performed on lung homogenates immediately following these O2 exposures are: ATP concentration, ATPase activity, succinic dehydrogenase activity, respiration and oxidative phosphorylation, ribonuclease activity, cathepsin activity, and total and mitochondrial protein levels. Following these studies the possible reversibility of the biochemical alterations will be determined by conducting experiments, using identical conditions, however allowing recovery periods of 1,3, and 5 days prior to making biochemical measurements to determine lung damage due to oxygen toxicity. The time required for return to normal biochemistry of the lung tissue will be determined, and also the sequence in which biochemical defects return to normal. At the time lung tissue is removed for bioassay, other sections of lung tissue will be obtained for pathological studies using light and electron microscopy. Biochemical changes will be correlated with the pathological changes observed in the lung tissues.